Although I do sometimes use 4F powder in the pan, the only reason I do is once upon a time I bought a pound of it.
Like many, I read about the wonders of using 4F to prime with.
How the gun would almost fire before the trigger was pulled.
How it would ignite even if the lock was poor and the flint would make few sparks....
Then I bought some and tried it out.
It seemed to work just like the 3Fg I had been using for years.
I can't tell a difference in speed.
A poor lock or flint that doesn't throw a good number of sparks still won't light the powder.
All in all, I was rather disappointed.
Except for use in little .32 caliber C&B pistols, most people agree it burns to fast for a main powder charge (although Hodgdon says it can be used for a starter load in small amounts under their synthetic powders).
With my new found knowledge I would recommend to a person buying powder for a flintlock, forget the 4F. Buy a extra pound of 3Fg.